Flashcards in Neurulation & Folding Deck (34):
What is the neural groove and what surrounds it?
By day 21, the neural plate starts to proliferate over the paraxial mesoderm, forming neural folds. These surround and form the neural groove at the midline
How does neural folding proceed?
Once the neural folds have formed, the ectodermal cells begin to pinch off the neural tube. This proceeds from the initial point of fusion in the cervical region both cranially and caudally
What are the rostral and caudal neuropores?
The end openings of the neural tube before it seals, the way it communicates with the amniotic cavity
When does fusion of the neural tube complete and what does that mark?
Completes by end of 4th week, neural tube is closed and broader at the cranial end (brain vesicles). CNS is developing
What are brain vesicles?
The three expanded regions at the cephalic end of the neural tube, including prosencephalon, mesencephalon, and rhombencephalon (primary vesicles) that give rise to different parts of brain
What is the prosencephalon and its two secondary subdivisions?
Forebrain brain vesicle
What is the mesencephalon?
Midbrain brain vesicle
What is the rhombencephalon and its two secondary subdivisions?
Hindbrain brain vesicle
What type of epithelium makes up the neural tube and what is special about it?
Pseudostratified columnar epithelium (neuroepithelium) which lies between both external and internal limiting membranes which are basal lamina
These give rise to all CNS neurons and glia
What are the zones of the neural tube epithelium?
Ventricular zone (inner) - proliferating neuronal progenitor cells
Mantle (intermediate) zone - migrating neuroblasts
Marginal zone - The axons of the developing neurons
Where does the neural crest arise from and up to what level?
Neural crest cells are a distinct layer in the edges of the neural folds in the cranial region. They are derived from neuroectoderm, and are produced up to the diencephalon level, but not telencephalon.
Why does telencephalon not produce neural crest cells?
They only produce neuroepithelial cells that form ectodermal placodes, which form small aggregates of specialized ectoderm within the surface ectoderm
What is the general fate of neural crest cells?
They undergo EMT and migrate to different locations in the embryo after losing contact with neuroectoderm
What do neural crest cells in the trunk region become?
Dorsal route: leads to skin, give rise to the melanocytes of epidermis and hair follicles.
Ventral route: They travel between anterior parts of somites and will form dorsal root ganglia, sympathetic ganglia, parasympathetic ganglia, para-aortic ganglia, Schwann cells, and C-cells of adrenal medulla
What do neural crest cells in the head become?
They leave before the tube is totally closed, and migrate tot their final destination within the developing pharyngeal arches, and most importantly the sensory neurons and glia of cranial ganglia
Where do ectodermal placodes appear? What is their role?
They are ectodermal thickenings that appear lateral to neural tube and crest. They have an important role in sensory organs, pituitary gland, teeth, and cranial ganglia (including sensory, along with neural crest cells)
What is the primary driving force of the sagittal head folding?
Rapid growth of cranial neural folds rising above surface ectoderm
How does relative positioning change in sagittal head folding?
The most cranial structures up to the buccopharyngeal membrane move ventrally and caudally. This makes the septum transversum, primordial heart, and intraembryonic coelom more caudal, in that order. The pivot point is the prosencephalon.
What forms the foregut?
During sagittal head folding the cranial end of the endoderm folds ventrally and caudally to form it. It is ended at the ectodermal derived stomodeum (the depression into the buccopharyngeal membrane) which will form the mouth
What does the septum transversum do after folding inward?
extends dorsally on each side of developing gut, separating pericardial cavity from the rest of the intraembryonic coelom (which is open to the extraembryonic coelom)
What are the three sections of the intraembryonic coelom after sagittal head folding?
1. Pericardial cavity - most cranial portion, closed off
2. Pericardio-peritoneal canals - two which run on each side of foregut dorsally.
3. Lateral edges of intraembryonic coelom which will form the peritoneal cavity
What is the pivot point for the sagittal folding of the tail region and what moves?
The end of the neural tube. The allantois and connecting stalk will now lie cranial to the cloacal membrane. The hindgut is formed
What forms the midgut?
The portion of the gut lying between the foregut and hindgut, but still connected to the yolk sac. Forming the midgut pinches off the yolk sac somewhat and forms the vitelline duct
What is the shape of the intraembryonic coelom after folding in sagittally?
It is like a horseshoe, closed at the end, which folds in on itself. So the top of the horseshoe (pericardial) is connected at both sides dorsally (pericardioperitoneal canals) and then runs backwards into the peritoneal cavity
After folding, where does the intraembryonic coelom lie in relation to the developing primordial heart?
What must happen before transverse folding can proceed?
The somatopleure and splanchnopleure grow ventrally towards the midline (early week 4)
What forms the definitive foregut/hindgut?
When the splanchnopleure fuses at the midline. The endodermal layer will be on the inside, lined by the splanchnic mesoderm.
What forms the peritoneal cavity?
The transverse folding brings ends of the somatopleure together, forming the ventral wall of the peritoneal cavity. It is lined with splanchnic mesoderm (around the gut), inside of outer wall is somatic mesoderm, and outside of outer wall is surface ectoderm
Why is the definitive foregut / hindgut different than the definitive midgut?
There is a space where the splanchnopleure and somatopleure cannot fuse because the yolk sac is preserved. The small connecting tube is called the vitelline duct
What is the primitive umbilical ring?
The oval line between the amnion and the surface ectoderm of the embryo
What structures pass through the umbilical ring?
Connecting stalk, allantois, and umbilical vessels (two arteries and one vein). Vitelline duct with vitelline vessels (from primitive hemangiogenesis), and canal connecting peritoneal cavity to extraembryonic cavity
How is the primitive umbilical cord formed?
Whenever the amnion begins to fill the chorionic cavity after formation of primitive umbilical ring. It is formed from vitelline duct and umbilical vessels, some intestinal loops (too big to fit inside abdominal cavity) and allantois remnants. Yolk sac remains connected to intestinal loops via vitelline duct
What happens by the third month of umbilical development?
The amnion has completely crowded out the chorionic cavity, and only the chorionic sac remains. The yolk sac shrinks, along with the vitelline duct and its vessels. It is degraded, and intestinal loops return to the abdominal cavity.